The present invention relates to an adsorption powder useful for the removal of metal and organic pollutants from gas streams. The adsorption powder is typically useful for treating solid waste contaminates, e.g. contaminated soil treatment by high efficiency incineration. More particularly, the invention relates to the capture of mercury and other metals, dioxins, furans and other organic compounds from high temperature, high moisture gas streams using an adsorption powder containing cupric chloride.
Strict standards exist for particulate and total mercury emissions by coal-fired power plants, petroleum refineries, chemical refineries, coal fired furnaces, trash burning facilities, incinerators, metallurgical operations, thermal treatment units and other particulate and mercury emitting facilities. These same restrictions apply to mercury vapor, which can enter the atmosphere as a result of low temperature thermal desorption (LTTD) treatment of contaminated soils.
These stringent standards exist in order to protect the environment and the community. When mercury-containing gases are released, the gases disperse and mercury is deposited over a wide area. The dispersed mercury can accumulate in the soil or water supplies, where it may be incorporated into the food chain. Mercury is extremely harmful to aquatic life and ultimately to the humans who consume mercury-contaminated plants and animals. It is necessary, therefore, to have a safe and effective method of eliminating mercury from the environment.
The problem of the capture and treatment of mercury vapor, typically in the context of coal-fired power plants and waste incinerators, has been previously considered. For example, U.S. Pat. No. 3,193,987 discloses passing mercury-containing vapor over activated carbon impregnated with a metal which forms an amalgam with mercury. U.S. Pat. No. 4,094,777 discloses passing a mercury-containing vapor over an adsorption mass consisting essentially of a support, sulfided copper and sulfided silver. U.S. Pat. No. 3,876,393 discloses passing mercury-containing vapors over activated carbon that has been impregnated with sulfuric acid. Selenium has also been used in the removal of mercury from a vapor. U.S. Pat. No. 3,786,619 discloses passing a mercury-containing gas over a mass containing as an active component, selenium, selenium sulfide or other selenium compounds. Electrostatic precipitators and various filters have traditionally been used for mercury removal, although complex apparatus have also been disclosed. (See e.g., U.S. Pat. Nos. 5,409,522 and 5,607,496.)
The problem of recapturing mercury from power plant gas streams is analogous to the need for recapturing mercury from incinerators that treat contaminated soils. A process currently in use at soil treatment facilities is known as low temperature thermal desorption (LTTD). LTTD is the main process by which contaminated soils are treated to remove mercury and other contaminants. In this process, contaminated soils are fed into a heating furnace, most commonly a rotary kiln/drum, where the soil is heated by conduction. The heating volatizes the soil components and when a thermal oxidizer is added, the components are oxidized to manageable gases, such as CO2, Cl2, NOx and SOx, where x is 1-3.
The hot gas stream is subsequently cooled. The stream may be quenched with water, which cools the stream and concurrently increases the moisture content. Although water quenching is a highly effective cooling method, this treatment increases the difficulty of removing mercury from the gas stream. The gas stream is further treated to reduce and remove metals, HCl, NOx and SOx using acid scrubbers, carbon beds, condensation units and through the addition of adsorption powders.
When adsorption powders are injected into the gas stream, mercury and other metals bind to moieties present in the powder, precipitating them from the gas stream. The powder-bound mercury is ultimately collected in a bag house for appropriate disposal, while the clean gas stream is exhausted to the outside atmosphere. The problem with standard LTTD methods is that some metals, such as mercury, are not removed from the stream at high efficiency and will move with the gas stream, ultimately into the environment. Other methods require the use of complex machinery and expensive adsorption beds. LTTD and other methods also suffer from the limitation that mercury removal from high moisture gas streams is much more difficult than mercury removal from dry streams.
Available adsorption powders remove organics, metals and other contaminants, but they do not effectively remove mercury. For example, one available powder (Sorbalite(trademark)) consisting of carbon, calcium hydroxide and sulfur removes HCl from a gas stream, but it removed only about 55-65% of the mercury. Another powder (WUELFRAsorb-C(trademark)) consisting of alcohol saturated lime and activated carbon is also inefficient at removing mercury.
Some powders include sulfur or iodine impregnated carbon. At temperatures of 75xc2x0 C. or less, sulfur or iodine impregnated carbon based powders show a 95% mercury removal efficiency, however, powders formulated with sulfur impregnated carbon require that the gas stream to which they are added is dry.
Lastly, the mercury removal efficiency of the powders described and other available powders is known to be very temperature dependent, placing an additional limitation on powder formulations.
Accordingly, there is a need in the industry for an adsorption powder that effectively removes metals and other organic compounds, in general, and mercury, in particular, from high temperature, high moisture gas streams generated by the incineration of contaminated soils, treatment of hazardous materials, combustion of coal and other mercury liberating sources. The powder must be inexpensive and easy to use. Ideally, such an adsorption powder can be employed at treatment facilities already in place and can take advantage of equipment already in position, without requiring retooling or reconfiguring existing equipment.
There is disclosed a process for removing mercury and organic compounds from gaseous streams using an adsorption powder, characterized as containing a carbon-based powder selected from the group consisting of coal carbons, wood carbons, graphite carbons, activated carbons, coconut shell carbons, peat carbons, petroleum cokes, synthetic polymers, the like, and combinations thereof, and an effective amount (about 3 to about 10 weight percent) of cupric chloride, the process being characterized by the steps of:
a) placing a solid phase mercury-containing contaminated soil feed into a rotary kiln/drum;
b) heating said kiln/drum containing said soil feed to form gaseous and solid components of the sample;
c) transferring the gaseous component of said soil feed to an exhaust cleaning unit/afterburner and the solid component of clean soil to a soil cooling unit;
d) heating the gaseous component of said contaminated soil feed in said exhaust cleaning unit/afterburner;
e) cooling the gaseous component of said contaminated soil feed;
f) adding the adsorption powder to the gaseous component;
g) transferring the powder-containing gaseous component to a baghouse; and
h) releasing the substantially mercury-free gaseous component of said sample to the atmosphere.
Optionally, sulfur, potassium iodide and permanganate, calcium hydroxide, and combinations thereof may be added to the powder.